1. Field of the Invention
The present invention relates to a measuring instrument and more particularly to a straightness measuring instrument.
2. Description of Related Art
With reference to FIG. 10, a conventional straightness measuring instrument 70 includes a base 71, two support units 72 and a measure unit 73. The support units 72 and the measure unit 73 are mounted on the base 71 and arranged parallelly. Each support unit 72 has two rollers 721. The measure unit 73 has an indicator 731. A workpiece 80 is placed on the rollers 721 of each support unit 72. The workpiece 80 is a long stick and has an identical diameter. In order to measure the straightness of the workpiece 80, the indicator 731 needs to be leaned against the periphery of the workpiece 80. If the workpiece 80 is bent or not straight enough, the pointer of the indicator 731 will be moved when the workpiece 80 is rotated by the support unit 72. Therefore, the conventional straightness measuring instrument 70 can measure the straightness of the workpiece 80.
If the workpiece 80 has different diameters at different parts of the workpiece 80, the workpiece 80 cannot be placed on the rollers 721 horizontally and be measured exactly by the measure unit 73. In addition, the rollers 721 of each support unit 72 are arranged separately and are hard to be moved. If the length of the workpiece 80 is shorter than a distance between the rollers 721, the workpiece 80 cannot be placed on the rollers 721.
To overcome the shortcomings of the conventional straightness measuring instrument, the present invention provides a straightness measuring instrument to mitigate or obviate the aforementioned problems.